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Combined rocket and ground observations of electron heating in the ionospheric F-layer
Planer.
SpaceSci., Vol.
40, No. 7. pp. 90-912,
0032-0633/92 $5.00+0.00 0 1992 Pergamon Press Ltd
1992
Printed in Great Britain.
COMBINED ROCKET AND GROUND OBSERVATIONS OF ELECTRON HEATING IN THE IONOSPHERIC F-LAYER K. R. SVENES,
B. N. MAEHLUM*
and J. TR0IM
Norwegian Defence Research Institute, P.O. Box 25, N-2007 Kjeller, Norway G. HOLMGREN Swedish lnstitute of Space Physics, Uppsala Division, S-7.5590 Uppsala, Sweden R. L. ARNOLDY
Institute for the Study of Barth, Oceans and Space, University of New Hampshire, Durham, NH 03824, U.S.A. U. P. L0VHAUG and M. T. RIETVELD EISCAT Scientific Association, N-9027 Ramtjordbotn,
Norway
C. HALL University of Tromse, N-9000 Tromser, Norway (Received infinal.form 28 February 1992) Abstract-Results from the combined ionospheric sounding rocket and incoherent scatter radar experiment NEED anon-M~weIljan Electron Energy Dist~bution} are presented. It is shown that the in-situ measurements of the electron temperature yiei-ded much higher values than the corresponding measurements obtained from the ground. A significant suprathermal electron population was also detected in the Player. It is suggested that these two observations are strongly related to each other. It is also shown that the suprathermal electron population was strongly field-aligned, and that the production region was above the rocket trajectory. This interaction was very likely to have been initiated by the aurora1 particle precipitation present during the whole experiment.
1.
INTRODUCI’ION
In recent years the polar D- and E-layers have been the subject of several studies, both from rockets and ground installations. By contrast, the F-layer has received comparably little attention. However, lately there has been a revival of interest in this most important region for studies of collisionless plasmas, HF communication and low-altitude satellite orbits. A key issue when considering models of the Fregion at aurora1 latitudes is always that of thermal stability. Accordingly, it is of considerable interest to examine the various electron temperature measurements which have been obtained in this region, since this is the species most sensitive to energy dissipation processes. This was the topic of a recent review by Oyama and Schlegel(l988). Based on several satellite and rocket experiments, they concluded that electron temperature anisotropy is a regular feature of these measurements of aurora1 F-region plasma. The following important points were found to be characteristics of the morphology of such anisotropies: *This article is dedicated to the memory of our dear colleague Bemt N. Maehhnn who died unexpectedly last autumn.
(1) The electron temperature is usually higher parallel to than perpendicular to the magnetic field. (2) The occurrence frequency of these anisotropies is greater at high latitudes. (3) The anisotropy seems to be greater at high altitudes. It is as yet not known which mechanisms are responsible for creating these anisotropies, but such interactions obviously have a significant place in the overall understanding of plasma processes occurring in the ionospheric F-region. It is also well known that electron tem~ratures inferred from space vehicle measurements are often significantly higher than temperatures inferred from ground instruments like incoherent scatter radars. Oyama and Schlegel (1988) suggested that this might well be ascribed to data interpretation problems connected with the effect of the above-mentioned electron temperature anisotropies. The idea is that the incoherent scatter radars situated at high latitudes are only in a position to sample the low temperature component at an angle to the field lines, whereas the electron probes onboard the space vehicles sampled the whole population and accordingly measured a higher 901
902
K.
R. SVENES
average temperature. However, as will be shown later, this discrepancy might well have other explanations too. In addition, there are also several other pieces of evidence for thermal instability processes operating in the aurora1 F-layer. In-situ observations from both rockets and satellites have repeatedly revealed the existence of non-Maxwellian electron energy distributions in relation to artificial beam injection experiments, see e.g. Papadopoulos and Szuszczewicz (1988), Szuszczewicz (1985) and Winckler (1980). During such “controlled” beam experiments, as well as at other times, it has also been noted that the ambient ionospheric electron temperature and density variations are often out of phase, see e.g. Jacobsen (1982) and Maehlum et al. (1984). Furthermore, it has even been observed that the induced temperature enhancements do not necessarily vary proportionally to the beam current input (Svenes et al., 1988). On a larger scale, examinations of the energy budget in the aurora1 F-layer may also be helpful in revealing any thermal instabilities. One such attempt has been reported by Lilensten et al. (1990). Their object was to use measurements of the primary aurora1 particles, obtained from the Viking satellite, as input to computer solutions of the appropriate MHD transport equations. The output profiles from these simulations were then compared with measurements from EISCAT obtained simultaneously with the satellite data. They concluded that the energy budget could be balanced well throughout the ionosphere, except for a region centered around about 250 km altitude where the observed heating rates were significantly greater than those which could be accounted for by the model used in the simulations, A similar discrepancy was noted by Svenes et al. (1992) when analyzing results from the combined rocket and incoherent scatter experiment NEED (Non-Maxwellian electron Energy Distribution). The inferred heating attributable to the primary particle current (measured from the rocket) was less by a factor of about four than that which was actually observed from the ground (measured by the incoherent scatter radar), This was tentatively attributed to the influence of wave-particle interactions in the Flayer. On the basis of the above-mentioned observations it seems clear that further studies specifically aimed at understan~ng the plasma dynamics behind these phenomena are appropriate. The NEED experiment was designed for just this purpose, namely to investigate possible thermal instability processes in the auroral F-region. With this goal in mind, it was decided to obtain simultaneous rocket and incoherent scatter
et ai.
radar measurements from the same region of space. This would enable us to untangle some of the microprocesses underlying the short-term variations of a macroscopic parameter like the electron temperature. In particular, it was of interest to determine whether processes other than collisions and thermal diffusion are of importance in the energy dissipation in the Flayer during aurora1 particle precipitation.
2. DESCRIPTION
OF THE EXPERIMENT
In this paper we will mostly be concerned with data from the NEED-I rocket which was launched from Andoya Rocket Range in the northern part of Norway at 1902 U.T. on 7 November, 1988. The launch occurred during a geomagnetically moderately disturbed period with the X-, Y- and Z-components having values of -281y, 1407 and -8Sy, respectively. Several parallel aurora1 arcs were observed optically across most of the sky, and the rocket passed above a series of these during the flight. As mentioned above, the NEED-ex~riment also involved the participation of an incoherent scatter radar, namely the EISCAT 933 MHz radar situated close to Tromso (see Folkestad et al., 1983). The purpose of this arrangement was for the radar to monitor the ionosphere prior to and during the flight to obtain a large-scale picture of the ionospheric background dynamics. Indeed, the near real-time display of these data at the launch site was actually used to indicate suitable launch conditions for the rocket. Just prior to launch, the ionospheric plasma was characterized by an increasing electron temperature in the F-layer and an increasing electron density in the E-layer. These conditions, which were a central part of the launch criteria, also persisted throughout the flight. A more thorough account of the EISCAT data from this period is given by Hall et al. (1989). Here, a full description of the operating mode of the EISCATfacilities during the flight is also provided. The geometry of the experiment is shown in Fig. 1 (from Hall et al., 1989). Due to the different locations of the launch site and the radar facility, EISCAT had to monitor the ionosphere at an angle to the geomagnetic field lines in order to cover the region of space containing the rocket trajectory. However, it is important to note that at the apogee point of the trajectory the radar monitored the same volume of space which was intersected by the rocket. Thus, in this region the various measurements obtained from the rocket and the radar should be directly comparable. As previously stated, a main goal of the experiment
Electron heating in the ionospheric F-layer
was to observe closely the energy dissipation processes in the F-layer. Hence, the payfoad was composed SO as to be able to monitor both the primary source of the F-region disturbance, i.e. the high energy auroraI electrons, the electrostatic and electromagnetic waves generated by these fast electrons and the modifications in the thermal plasma resulting from any such interactions. The complete list of the participating institutions aud a short description of their ~nst~ments 1s given in Table 1. Unfortunately, due to a maIfunction, the payload did not despin properly. As a consequence of this, some of the antennas broke off and no wave data were obtained during the flight. Thus, in this paper we will concentrate on the analysis of the primary particle
903
data and the measurements obtained from the plasma probes. In addition, EISCAT data will of course also be frequently referred to. 3. THE BACKGROUND MEASUREMENTS
The ionosphere was monitored from the EISCAT facility in the hours prior to the launch. Figure 2 shows the electron temperature (upper part) and eIe;ctron density (lower part) measurements from the period one hour prior to the launch and onwards to just after the impact, Since the antenna was tilted to an angle of about 33” to the geomagnetic field lines, both parameters are plotted as functions of range {distance from antenna). Hence, it is of importance to note that
K. R. SVENE~et al.
904
TABLE1.PAYLOADINSTRUMENTATION Suprathermal electron spectrometer
Norwegian Defence Research Establishment
Electrons, directional measurements, &I5 eV. Low energy particles
University of New Hampshire, U.S.A.
Aurora1 electrons in the range 15 eV-20 keV. Solid-state detectors
University of Bergen, Norway
Aurora1 electrons and ions above 25 keV. Langmuir probe
Swedish Institute of Space Research, Uppsala Division
Relative changes in the ion density. At intervals, also electron temperature and density. Electric field detector
Goddard Space Flight Center, U.S.A.
Three-axis d.c. E-field, broadband ELF/VLF waves. Spectrum analyzer
University of Oslo, Norway
Waves between 50 Hz and 5 MHz. Quadrupole probe
Norwegian Defence Research Establishment
Electron density. HF-receivers
Norwegian Defence Research Establishment
Electrostatic waves, l-10 MHz.
these values do not describe the ionosphere in terms of altitude profiles in the ordinary sense. The apogee point was reached at approximately 19:07 U.T. and was located at a range of 371 km. This corresponds to an altitude of 322 km. The launch occurred after a period of time when the ionosphere was rather quiet (18:00 U.T. to almost 19:00 U.T.). As shown in the figure, there seems to have been an anti-correlation between the electron temperature and density in the F-region during this period. The temperature varied between about 1400 and 2000 K, while the density was in the range 25 x 10” rne3, which are about average values at these altitudes. However, as seen from Fig. 2, the electron temperature attained high values at times when the electron density was at low values. This situation repeated itself several times in the period leading up to the launch. Just before 19:00 U.T. there was a particle precipitation onset, as is seen from the sharp electron density increase in the E-layer. At the same time, the electron temperature in the F-layer, which had already been on the increase for a few minutes, increased further, reaching values of more than 2600 K. Since the launch criteria were now fulfilled the rocket was launched at 19:02 U.T., and the flight lasted for almost 10 min. Although the electron temperature decreased slightly during this time, the ionospheric situation remained more or less the same throughout the whole flight. These observations should be compared to the measurements carried out from the rocket itself. In Fig. 3 the electron temperature (right part) and density (left part) are plotted as functions of altitude. The two curves in each of these panels are related to the upleg and downleg of the flight. As is seen, these data
in themselves contain no great surprises. The density profile shows, as expected, a distinct E-layer maximum at about 100 km, before falling off to a minimum value around 200 km. The density then increases again to form the F-layer peak. The temperature, on the other hand, increases more or less monotonically with altitude. Primary particle measurements from the NEED-l rocket show that there was a more or less continuous precipitation of aurora1 particles during the whole flight. The particle spectra exhibited the “usual” auroral signature with an intensity peak at some keV, moving to higher energies (more than 25 keV) at apogee before decreasing again in the latter half of the flight. The general intensity increased as well during that period of time. It should also be noted that the precipitating particles were clearly field-aligned, especially at energies around a few keV, for significant parts of the flight. Since the rocket at the time of apogee passage was travelling through the same volume of space as that monitored by EISCAT, the measurements obtained around this time should, in principle, be directly comparable to each other. In Fig. 4 the electron temperature obtained from the rocket measurements (dots) are compared to the electron temperature obtained from the EISCAT measurements (crosses) at different altitudes. As is seen, there is a better agreement between the two curves at lower altitudes, where the measurements were obtained from different regions, than around the apogee. There, the in-situ measurements indicated an electron temperature of more than 3000 K at that time. The data obtained from the ground, on the other hand, yielded a corresponding value of around 2000 K. Even though these measurements were obtained by very different
Electron heating in the ionospheric F-Layer
L
RANGE
(km)
Te f@3 2600 2450 2300 2150 2000 1850 ??OO t5sO WOO 1250 1100 950 800
Ne fm”)
1. OE+t2
i
FIG, 2. ELECTRON TEMPERATURE (K),w PART,PLOTIEDASPUNCTIONOFDISTANCE
Tm
6. 8E+11 4. 6E+ll 3. 2E+lf 2, 2E+ll 1. 5E+lf f. OE+lf 6. 8E+ f0 4.6E+lO 3. 2E+lO 2. ZE+lO 1. 5E+lO 1. OE+lO
UPPER PARC, AND ELECTRON DENXTY (m-‘), IN 17% LOWER (km) FROMT~AAWTENNADURINGTHITEVENT 7 NOVEMBER, 1988. Intensity scales are given to the right of the respective plots. The elevation angle of the antenna was 60.3”.
907
Electron heating in the ionospheric F-layer I
I
I
I
NEED-II flight as well. During that flight the ionospheric conditions were rather quiet with only a small number of precipitating particles present. In this case the measurements carried out from the rocket agreed very well with the EISCAT data. Thus, it seems likely that the discrepancies between the rocket and ground observations during the NEED-I flight were related to the plasma conditions inside the particle precipitation region rather than any instrumental effects. Consequently, on the basis of the previous argument, it seems reasonable to direct the investigation of these measurements towards the mechanism which can most readily be related to the actual plasma environment during crossings of precipitation regions. Hence, in the following we will turn our attention to the question of the influence of suprathermal electron populations on the electron temperature measurements.
4. INTERPRETATION
OF THE AMBIENT
ELECTRON
MEASUREMENTS
i---r-x
i
CLC
Log
Ne.
Te. eV
cm-3
FIG. 3. ELECTRON DENSITY (cm') IN THE LEFT PART AND ELECTRON TEMPERATURE (ev) IN THE RIGHT PART, PLOlTED AS FUNCTIONS
OF ALTITUDE.
Data were obtained by the Langmuir probe onboard the NEED-I rocket.
techniques, a discrepancy of a factor of two warrants some attention. Several comparisons of rocket and incoherent scatter measurements of the electron temperature have been performed previously, see e.g. Oyama and Schlegel (1988) and references therein. These have resulted in diverging conclusions with the in-situ measurements sometimes agreeing with the remote measurements, but just as often turning out to be significantly higher than the ground measurements. Various explanations have been put forward to explain these differences, including electron temperature anisotropies, probe surface phenomena and the influence of non-Maxwellian velocity distributions. In this connection, it is of interest to note that another comparison of electron temperature measurements from rockets and from EISCAT has already been presented by Schlegel and Oyama (1987). These authors concluded that the measurements were in good agreement outside an aurora1 arc encountered by the rocket late in the flight. This is in agreement with our experience from the
Since it was expected that the electron population encountered during this flight might be highly nonthermal, the rocket was equipped with a suprathermal electron spectrometer (SES) mounted on a 16 cm long boom. This instrument was operative at altitudes above about 90 km both on the upleg and the downleg. However, for a short period on the upleg, an electronic malfunction rendered the measurements from the height interval between about 150 and 220 km useless. The same event also disabled part of the dynamic range of the sensors, but still left the instrument capable of obtaining measurements with a high enough quality to give a fairly good representation of the ambient electron population. The instrument consists of two directional electron probes with time-varying retarding potential analyzers. These were swept up and down in voltage between - 14.5 and + 1.5 V, relative to the SES structure, resulting in a time resolution of 0.2 s for these measurements. This corresponded to a spatial resolution of about 71 m at the time of the apogee passage. The whole SES structure was biased to + 1.8 V relative to the payload in order to reduce the effects of any negative vehicle charging. A schematic diagram of the sensor is shown in Fig. 5. In order to appreciate the significance of this design, it is important to understand the consequences of some of the details of the particular analysis technique used in this experiment. The two electron probes were both built into a metallic sensor box with a circular aperture defining the opening angle of each probe. The electrons would then enter the analyzer through
908
K. R. SVENES et al.
200-
0
Rc.4.
1000
X -
Te, EISCAT
.
Te, ROCKET
2000
-
3000
ELECTRON TEMPERATUREMEASUREMENTSOBTAINED
4000
DURING THE NEED-I
K
CAMPAIGN.
The curve marked by crosses is based on incoherent scatter radar measurements (EISCAT), the curve marked by dots is based on Langmuir probe measurements from the rocket. Notice the large differences in the temperature measurements (ca 1000 K) at the apogee point of the trajectory. This is especially noteworthy since these measurements, in contrast to lower altitudes, were obtained from the same volume of space.
a combination of their own momentum and the influence of the electric field induced by the bias potential of the sensor box. Once inside the box the electrons are immediately subjected to an analysis field in order to differentiate between their various energies. At this point, however, these electrons are no longer in contact with the ambient plasma. Hence, their energy spectra will always reflect both the distribution
SIMPLIFIED
DIAGRAM
inherent to the plasma itself and the result of its interaction with the sensor box. The above argument applies regardless of whether the sensor box is positive or negative relative to the plasma potential. However, the interpretation of the resulting energy spectra is crucially dependent on this relationship. If the box is at a positive potential relative to the plasma, the resulting energy spectrum will
OF SES - SENSOR ROCKET STRUCTURE
RETARDING
\ -14.5
I II
POTENTIAL
+ 1.5
/
INDIVIDUALLY (SWEEPING) BIASED
BAFFLES
FIG. 5.A SCHEMATIC DIAGRAM OFTHESES-SENSOR BOX. The electrons enter the sensor from the left, and are then submitted to an analysis field inside the box. Since the box in effect acts as a Faraday cage, the electrons have no contact with the surrounding plasma once they are inside the sensor. The limited size of the opening in the box gives the sensor an effective opening angle of about 60”.
Electron heating in the ionospheric F-iayer
exhibit the characteristics of the internal distribution of the electron population augmented by the energy imparted to it by the ~s~~vely biased box. On the 5th hand, if&e box is negative relative to the @asma onfy those efeetrons with enough energy to penetrate the sheath surrounding the box will then be able to enter it and subsequently become analyzed there. Hence, the resultmg spectrum is then in reality only a part of the total electron d~s~bu~~on of the ambient piasma. Usually9 a precise knowledge of fhe bias applied to the sensor box (relative to the payload) and the charging of the main payload (relative to the plasma) will enable us to determine the effective potential of the SES relative to the plasma potential. Unfore
F-REGION
nA
&mate@, &ring the ftight of NEED-I Sxse criteria were not fulfilled. The languor probe did not seem
to indicate any major charging of the payload structure during the flight. However, certain problems with the interpretation of these instruments at present seem to indicate that no unique rocket ground potential conid be established. In addition, the same electronic ma~fun&t~on which hampered the SES-meas~~m~nts during part of the flight probably also left the sensor box in a state where its bias relative to the payload was indeterminate. However, there exists circumstantial evidence which may still guide us to fairly reasonable canciusions aborts the So-m~su~ments~ This is in part, based on the certain properties of integral current measurements and partly on supporting measurements by the Langmuir probe. It is well known that, while the sweep potential is in the repulsive regime of the plasma electrons and the electrons are assumed io be Ma~we~~~an~~t~bnt~, the resulting cnrrentvoltage curves represent the farnihar Boftzmann relation for electrons. ‘This interpretation was first. described by Langmuir and Mott-Smith Jr (1924) in detail. By comparing the actual measured curves with estimated distributions, based on the above assumptions and using plasma parameters obtained by the Langmuir prtllx, it should be possiMeP in principle, to draw some canclusions about the electron spectra obtained by the SES as well. The most striking feature of the SES measurements is the fundamental difference between data obtained in the F-layer and the E-layer. This is illustrated in Fig. 6. fn this figure the actual data sets ~~~~~~js~~~ are plotted together with estimated distributions (triangles) in order to relate the measurements indirectly to the ambient plasma parameters. The lower part shows a spectrum obtained at an altitude of 120 km. This is representative of the measurements inside the E-layer both on the upieg and the downleg. The ear-
E-REGION nA 4.0
3.5 1 :
*=2.
I 3.0
*@tm-’
T=llWK
1
t 1
ev
0.D 0.0
0.2
0.4
0.6
0.8
1.5
1.2
1.4
FIG. 6. TWO GLECTRON ENERGY SPECTRA OBTAINED I>UBfNG FLIGHT OF NEED-1 COMPARED WITH CORRESmNDING
THE
SiMUu~D
SPECTRA.
marked by after&k& wtiie triangfcs. 1~ the upgxx oa~t ofthe figure is dis&aved data From the Flayer obtained it an altitudeof 250 km, and in the lower part data from the E-layer obtained at an altitude of 120 km. The important point to note here is that the spectrum obtained in the F* layer indicates a much wider electron energy distribution than the one from the E-layer.
The
act&
m~swd
the sinmlated
spxti
spectraare
zs%
givenby
responding simulated spectrum is based on the assumption that the electron population was Maxwellian distributed with a temperature of 1100 K and a density of 2 x 10” m- 3 (as measured by the Langmuir probe].
PLO
K. R. Scenes et at.
Comparing this with the spectrum displayed in the upper part of Fig. 6 immediately focuses on the qualitative difference between the two cases. This spectrum was obtained at an altitude of 250 km and is typical for the m~su~men~ made during the whoie passage through the F-layer. The corresponding simulated spectrum is this time based on the assumption that the electron population was bi-Maxwellian distributed with a thermal population having a temperature of 2500 K and a density of I x 10” me3 (again as measured by the Langmuir probe). In order to describe the high-energy part of this spectrum, it was necessary to introduce a suprathermal tail as well, having a temperature of about 6500 K and a density of about 5% of the thermal population. This was done on the basis of a best tit to the data points. The fact that only very tow values (currents) were actually observed in the low-energy portion of the spectrum is best understood by assuming that the sensor box was at an effective negative potential relative to the plasma during this part of the flight. This is based on the following argument. First suppose that the spectrum in the upper part of Fig. 6 realty represented a thermal electron distribution. Fitting this spectrum to a Maxwellian distribution would then yield a plasma density some two orders of magnitude lower than that which was observed both by the Langmuir probe and by EISCAT. This is in itself a highly unlikely interpretation. As pointed out above, there is also the matter of the considerable energy broadening of the spectra when comparing sweeps obtained in the F-layer with those obtained in the E-layer. The fact that the spectrum in the upper part of Fig. 6 contains electrons with up to 0.5 eY more energy than the spectrum in the lower part argues that it is the high-energy portion of the electron distribution which now dominates the measurements (remember that these are integral measurements). It is also noteworthy that the Langmuir probe and the SES yielded similar results in the E-layer on both upleg and downleg, thereby indicating that the SES low-energy measurements functioned properly also during the passage through the Flayer. Hence, we feel justified in assuming that the spectra obtained here represent the suprathermal part of the ambient electron distribution. Adopting this view, it is then possible to monitor the temporal development of these measurements for characteristic trends in the suprathermal electron population. In Fig. 7 we have plotted the angle between the upward pointing SES sensor and the local geomagnetic field (upper part) and the total current measured during each sweep by that sensor (iower part) for a period of time during the downleg.
The altitude was somewhat more than 300 km at this point. It is evident that the current was modulated by the angle between the sensor and the geomagnetic geld. The maximum current was always obtained when the sensor was most field-aligned. This was a regular feature during that part of the tlight in which the rocket was outside the E-layer. Furthermore, the same trend was seen in the data collected by the downward-looking sensor. However, these currents were always smaller than those collected by the upward-looking sensor. In Fact, the ratio of the current in the upwardlooking sensor to that of the downward-looking sensor amounted to more than 2 at times. Thus, if we assume as argued above, that the measurements are indicative of a suprathermal eketron population, we are left with the following interpretation. There seems to have been created a quite significant field-aligned suprathermal electron population in a source region somewhere above the rocket apogee (which was at an altitude of 322 km). Based on the current intensities measured by the SES, it seems likely that this population, at times, must have reached about 5% of the density of the regular thermal electron population in the P-region. 5. DISCUSSION
AND CONCLUSION
Based on the assumption that the SES was in fact able to measure a part of the suprathermal electron population inside the F-layer, we are now in a position to reconcile several of the other observations made during the NEED-I flight. These data may be explicable by any of several models when viewed separately, but we feel that the existence of a suprathermal electron component in the ambient plasma constitutes the only mechanism able to inliuence all of these measurements in a self-consistent manner. First of all we have the huge differences in the currents collected by the SES sensors in the F-layer and the E-layer (illustrated in Fig. 6). This was probably due to sensors being charged negatively retative to the plasma potential when the rocket was outside the relatively high density plasma in the E-layer, and consequently only the high energy part of the electron spectrum was then measured. The alternative explanation would have been that the SES measured a significantly enhanced electron temperature and a plasma density almost two orders of magnitude lower than that which was observed by both EXSCAT and the ion probe on the same rocket. This explanation seems highly unlikely since the SES and the ion probe made very similar measurements in the E-layer both on the upleg and the dowmeg.
911
Electron heating in the ionospheric F-layer
350.000
351.000
352.000
354.000
353.000
..__......... 2.505-10 ~ .._.__._____._.....................................................................~.......................................... ~
i.SOE-10 350.000
I 351.OOo
FIG. 7. IN THE UPPER PART IS SHOWN
”
352.000
I
”
”
353.000
THE ANGLE BETWEEN THE UPWARD LOCALGEOMAGNETICFIELDLINE.
POINTING
i
35r1.000 SES SENSOR
AND THE
In the lower part is shown the total current measured during each sweep of that sensor. Both of these parameters are plotted as functions of time of flight, and the corresponding data were obtained in the Flayer at an altitude of about 300 km.
Secondly, there is the discrepancy between the EISCAT and the in-situ electron temperature measurements around the time of the rocket apogee passage (shown in Fig. 4). At this time the observations from both the ground and the rocket were in the same volume of plasma, but the electron temperature measured from the rocket was still well in excess of that observed by the EISCAT radar. As mentioned previously, this kind of discrepancy has been observed several times before and tentatively attributed to various mechanisms. However, Hoegy (197 1) treated this problem theoretically and showed that large temperature differences would occur in the presence of significant suprathermal electron populations. In such a situation the temperature deduced from probes would be the higher, as was indeed the case during the flight of the NEED-I rocket. Finally, there is also the question of the enhanced heating rates during this event, noted by Svenes et al. (1992). Since the observed ionosphere electron temperature increase during the flight was significantly higher than that which could be accounted for by collisional heating from the precipitating particles, the extra energy dissipation in the F-region was here tentatively ascribed to some sort of wave-particle
interaction. Such interactions would of course leave traces in the form of an enhanced suprathermal electron population. Furthermore, wave modes likely to be involved in such interactions, like Langmuir waves or the field-aligned mode of the lower hybrid waves, have repeatedly been observed in the aurora1 region, see e.g. McFadden et al. (1986), Bering et al. (1987) and references therein. In such cases the amount of suprathermal electrons would have been greatest parallel to the geomagnetic field lines. This too, is in accordance with the NEED-I observations. In conclusion, we feel that there is enough evidence to establish that a significant suprathermal electron population was present in the F-region during the flight of the NEED-I rocket. Unfortunately, there were no wave measurements to confirm any correlation with the particle measurements. However, as shown above, there is circumstantial evidence for assuming that the suprathermal electron populations may have been the result of wave-particle interactions. In any case, this population could very well be a key to the understanding of the mechanisms responsible for the energy dissipation obviously occurring inside the F-layer during aurora1 particle precipitation. Thus, the study of this question would
K. R. Svar*IES et
912
probably help to explain several interesting energy transfer mechanisms active in the ionospheric plasma and should, as such, warrant further work both theoretically and experimentally. Acknowledgements-The authors appreciate the excellent work of the payload integration crew headed by 0. Kristiansen. This project was financially supported by the Royal Norwegian Council for Scientific Research.
REFERENCES
a/.
Lilensten, J., Fontaine, D., Kofman, W., Eliasson, L., Lathuillere, C. and Oran, E. S. (1990) Electron energy budget in the high-latitude ionosphere during Viking/EISCAT coordinated measurements. J. geophys. Res. 95,608l. Maehlum, B. N., Hansen, T., Brekke, A., Holt, 0. and Folkestad, K. (1984) Preliminary results from a study of the F-region heating during an intense aurora. J. afm. terr. Phys. 46, 619.
McFadden, J. P., Carlson, C. W. and Boehm, M. H. (1986) High-frequency waves generated by aurora1 electrons. J. geophys. Res. 91, 12,079. Oyama, K-i. and Schlegel, K. (1988) Observations of electron temperature anisotropy in the ionosphere : a review. Ann. Geophys. 6, 389.
Bering, E. A., Maggs, J. E. and Anderson, H. R. (1987) The plasma wave environment of an amoral arc : 3. VLF hiss. J. geophys. Res. 92, 7581. Folkestad, K., Hagfors, T. and Westerlund, S. (1983) EISCAT: an update description of technical characteristics and operational capabilities. Radio Sci. 18, 867. Hall, C., Brekke, A., Rietveld, M. T., Lovhaug, U. P. and Maehlum, B. N. (1989) Background electrodynamics measured by EISCAT during the NEED-campaign. In Proc. Ninth ESAIPAC Symp. on European Rocket and Balloon Programmes and Related Research, Lahnstein, Germany,
p. 153, ESA Publication No. SP-291. Hoegy, W. R. (1971) Probe and radar electron temperatures in an isotropic nonequilibrium plasma. J. geophys. Res. 76, 8333. Jacobsen, T. A. (1982) Observations of plasma heating effects in the ionosphere by a rocket borne electron accelerator. In Artificial Particle Beams in Space Plasma Studies (Edited by Grandal, B.), p. 175. Plenum Press, London. Lanamuir, I. and Mott-Smith Jr. H. (1924) Studies ofelectric discharges in gases at low pressures. ken. Elec. Rev. 27, 449.
Papadopoulos, K. and Szuszczewicz, E. P. (1988) Current understanding and issues on electron beam injection in space. Adv. Space Res. 8, 101. Schlegel, K. and Oyama, K-i. (1987) Remote and in-situ olasma measurements durina the CAESAR fliaht. In Proc. ‘Eighth ESAIPAC Symp. on-European Rocket&d Balloon Programmes and Related Research. Sunne, Sweden, p. 3 15,
ESA Publication No. SP-270. Svenes, K. R., Maehlum, B. N. Lovhaug, U. P., Rietveld, M. T. and Hall, C. (1992) An investigation of two different electron heating events in the ionospheric F-layer. Adv. Space Res. 12,241.
Svenes, K. R., Troim, J., Friedrich, M., Torkar, K. M. and Holmgren, G. (1988) Ionospheric plasma measurements from the accelerator rocket MAIMIK. Planet. Space Sci. 36, 1509.
Szuszczewicz, E. P. (1985) Controlled electron beam experiments in space and supporting laboratory experiments : a review. J. arm. terr. Phys. 47, 1189. Winckler, J. R. (1980) The application of artificial electron beams to magnetospheric research. Rev. Geophys. Space Phys. 18, 659.
SpaceSci., Vol.
40, No. 7. pp. 90-912,
0032-0633/92 $5.00+0.00 0 1992 Pergamon Press Ltd
1992
Printed in Great Britain.
COMBINED ROCKET AND GROUND OBSERVATIONS OF ELECTRON HEATING IN THE IONOSPHERIC F-LAYER K. R. SVENES,
B. N. MAEHLUM*
and J. TR0IM
Norwegian Defence Research Institute, P.O. Box 25, N-2007 Kjeller, Norway G. HOLMGREN Swedish lnstitute of Space Physics, Uppsala Division, S-7.5590 Uppsala, Sweden R. L. ARNOLDY
Institute for the Study of Barth, Oceans and Space, University of New Hampshire, Durham, NH 03824, U.S.A. U. P. L0VHAUG and M. T. RIETVELD EISCAT Scientific Association, N-9027 Ramtjordbotn,
Norway
C. HALL University of Tromse, N-9000 Tromser, Norway (Received infinal.form 28 February 1992) Abstract-Results from the combined ionospheric sounding rocket and incoherent scatter radar experiment NEED anon-M~weIljan Electron Energy Dist~bution} are presented. It is shown that the in-situ measurements of the electron temperature yiei-ded much higher values than the corresponding measurements obtained from the ground. A significant suprathermal electron population was also detected in the Player. It is suggested that these two observations are strongly related to each other. It is also shown that the suprathermal electron population was strongly field-aligned, and that the production region was above the rocket trajectory. This interaction was very likely to have been initiated by the aurora1 particle precipitation present during the whole experiment.
1.
INTRODUCI’ION
In recent years the polar D- and E-layers have been the subject of several studies, both from rockets and ground installations. By contrast, the F-layer has received comparably little attention. However, lately there has been a revival of interest in this most important region for studies of collisionless plasmas, HF communication and low-altitude satellite orbits. A key issue when considering models of the Fregion at aurora1 latitudes is always that of thermal stability. Accordingly, it is of considerable interest to examine the various electron temperature measurements which have been obtained in this region, since this is the species most sensitive to energy dissipation processes. This was the topic of a recent review by Oyama and Schlegel(l988). Based on several satellite and rocket experiments, they concluded that electron temperature anisotropy is a regular feature of these measurements of aurora1 F-region plasma. The following important points were found to be characteristics of the morphology of such anisotropies: *This article is dedicated to the memory of our dear colleague Bemt N. Maehhnn who died unexpectedly last autumn.
(1) The electron temperature is usually higher parallel to than perpendicular to the magnetic field. (2) The occurrence frequency of these anisotropies is greater at high latitudes. (3) The anisotropy seems to be greater at high altitudes. It is as yet not known which mechanisms are responsible for creating these anisotropies, but such interactions obviously have a significant place in the overall understanding of plasma processes occurring in the ionospheric F-region. It is also well known that electron tem~ratures inferred from space vehicle measurements are often significantly higher than temperatures inferred from ground instruments like incoherent scatter radars. Oyama and Schlegel (1988) suggested that this might well be ascribed to data interpretation problems connected with the effect of the above-mentioned electron temperature anisotropies. The idea is that the incoherent scatter radars situated at high latitudes are only in a position to sample the low temperature component at an angle to the field lines, whereas the electron probes onboard the space vehicles sampled the whole population and accordingly measured a higher 901
902
K.
R. SVENES
average temperature. However, as will be shown later, this discrepancy might well have other explanations too. In addition, there are also several other pieces of evidence for thermal instability processes operating in the aurora1 F-layer. In-situ observations from both rockets and satellites have repeatedly revealed the existence of non-Maxwellian electron energy distributions in relation to artificial beam injection experiments, see e.g. Papadopoulos and Szuszczewicz (1988), Szuszczewicz (1985) and Winckler (1980). During such “controlled” beam experiments, as well as at other times, it has also been noted that the ambient ionospheric electron temperature and density variations are often out of phase, see e.g. Jacobsen (1982) and Maehlum et al. (1984). Furthermore, it has even been observed that the induced temperature enhancements do not necessarily vary proportionally to the beam current input (Svenes et al., 1988). On a larger scale, examinations of the energy budget in the aurora1 F-layer may also be helpful in revealing any thermal instabilities. One such attempt has been reported by Lilensten et al. (1990). Their object was to use measurements of the primary aurora1 particles, obtained from the Viking satellite, as input to computer solutions of the appropriate MHD transport equations. The output profiles from these simulations were then compared with measurements from EISCAT obtained simultaneously with the satellite data. They concluded that the energy budget could be balanced well throughout the ionosphere, except for a region centered around about 250 km altitude where the observed heating rates were significantly greater than those which could be accounted for by the model used in the simulations, A similar discrepancy was noted by Svenes et al. (1992) when analyzing results from the combined rocket and incoherent scatter experiment NEED (Non-Maxwellian electron Energy Distribution). The inferred heating attributable to the primary particle current (measured from the rocket) was less by a factor of about four than that which was actually observed from the ground (measured by the incoherent scatter radar), This was tentatively attributed to the influence of wave-particle interactions in the Flayer. On the basis of the above-mentioned observations it seems clear that further studies specifically aimed at understan~ng the plasma dynamics behind these phenomena are appropriate. The NEED experiment was designed for just this purpose, namely to investigate possible thermal instability processes in the auroral F-region. With this goal in mind, it was decided to obtain simultaneous rocket and incoherent scatter
et ai.
radar measurements from the same region of space. This would enable us to untangle some of the microprocesses underlying the short-term variations of a macroscopic parameter like the electron temperature. In particular, it was of interest to determine whether processes other than collisions and thermal diffusion are of importance in the energy dissipation in the Flayer during aurora1 particle precipitation.
2. DESCRIPTION
OF THE EXPERIMENT
In this paper we will mostly be concerned with data from the NEED-I rocket which was launched from Andoya Rocket Range in the northern part of Norway at 1902 U.T. on 7 November, 1988. The launch occurred during a geomagnetically moderately disturbed period with the X-, Y- and Z-components having values of -281y, 1407 and -8Sy, respectively. Several parallel aurora1 arcs were observed optically across most of the sky, and the rocket passed above a series of these during the flight. As mentioned above, the NEED-ex~riment also involved the participation of an incoherent scatter radar, namely the EISCAT 933 MHz radar situated close to Tromso (see Folkestad et al., 1983). The purpose of this arrangement was for the radar to monitor the ionosphere prior to and during the flight to obtain a large-scale picture of the ionospheric background dynamics. Indeed, the near real-time display of these data at the launch site was actually used to indicate suitable launch conditions for the rocket. Just prior to launch, the ionospheric plasma was characterized by an increasing electron temperature in the F-layer and an increasing electron density in the E-layer. These conditions, which were a central part of the launch criteria, also persisted throughout the flight. A more thorough account of the EISCAT data from this period is given by Hall et al. (1989). Here, a full description of the operating mode of the EISCATfacilities during the flight is also provided. The geometry of the experiment is shown in Fig. 1 (from Hall et al., 1989). Due to the different locations of the launch site and the radar facility, EISCAT had to monitor the ionosphere at an angle to the geomagnetic field lines in order to cover the region of space containing the rocket trajectory. However, it is important to note that at the apogee point of the trajectory the radar monitored the same volume of space which was intersected by the rocket. Thus, in this region the various measurements obtained from the rocket and the radar should be directly comparable. As previously stated, a main goal of the experiment
Electron heating in the ionospheric F-layer
was to observe closely the energy dissipation processes in the F-layer. Hence, the payfoad was composed SO as to be able to monitor both the primary source of the F-region disturbance, i.e. the high energy auroraI electrons, the electrostatic and electromagnetic waves generated by these fast electrons and the modifications in the thermal plasma resulting from any such interactions. The complete list of the participating institutions aud a short description of their ~nst~ments 1s given in Table 1. Unfortunately, due to a maIfunction, the payload did not despin properly. As a consequence of this, some of the antennas broke off and no wave data were obtained during the flight. Thus, in this paper we will concentrate on the analysis of the primary particle
903
data and the measurements obtained from the plasma probes. In addition, EISCAT data will of course also be frequently referred to. 3. THE BACKGROUND MEASUREMENTS
The ionosphere was monitored from the EISCAT facility in the hours prior to the launch. Figure 2 shows the electron temperature (upper part) and eIe;ctron density (lower part) measurements from the period one hour prior to the launch and onwards to just after the impact, Since the antenna was tilted to an angle of about 33” to the geomagnetic field lines, both parameters are plotted as functions of range {distance from antenna). Hence, it is of importance to note that
K. R. SVENE~et al.
904
TABLE1.PAYLOADINSTRUMENTATION Suprathermal electron spectrometer
Norwegian Defence Research Establishment
Electrons, directional measurements, &I5 eV. Low energy particles
University of New Hampshire, U.S.A.
Aurora1 electrons in the range 15 eV-20 keV. Solid-state detectors
University of Bergen, Norway
Aurora1 electrons and ions above 25 keV. Langmuir probe
Swedish Institute of Space Research, Uppsala Division
Relative changes in the ion density. At intervals, also electron temperature and density. Electric field detector
Goddard Space Flight Center, U.S.A.
Three-axis d.c. E-field, broadband ELF/VLF waves. Spectrum analyzer
University of Oslo, Norway
Waves between 50 Hz and 5 MHz. Quadrupole probe
Norwegian Defence Research Establishment
Electron density. HF-receivers
Norwegian Defence Research Establishment
Electrostatic waves, l-10 MHz.
these values do not describe the ionosphere in terms of altitude profiles in the ordinary sense. The apogee point was reached at approximately 19:07 U.T. and was located at a range of 371 km. This corresponds to an altitude of 322 km. The launch occurred after a period of time when the ionosphere was rather quiet (18:00 U.T. to almost 19:00 U.T.). As shown in the figure, there seems to have been an anti-correlation between the electron temperature and density in the F-region during this period. The temperature varied between about 1400 and 2000 K, while the density was in the range 25 x 10” rne3, which are about average values at these altitudes. However, as seen from Fig. 2, the electron temperature attained high values at times when the electron density was at low values. This situation repeated itself several times in the period leading up to the launch. Just before 19:00 U.T. there was a particle precipitation onset, as is seen from the sharp electron density increase in the E-layer. At the same time, the electron temperature in the F-layer, which had already been on the increase for a few minutes, increased further, reaching values of more than 2600 K. Since the launch criteria were now fulfilled the rocket was launched at 19:02 U.T., and the flight lasted for almost 10 min. Although the electron temperature decreased slightly during this time, the ionospheric situation remained more or less the same throughout the whole flight. These observations should be compared to the measurements carried out from the rocket itself. In Fig. 3 the electron temperature (right part) and density (left part) are plotted as functions of altitude. The two curves in each of these panels are related to the upleg and downleg of the flight. As is seen, these data
in themselves contain no great surprises. The density profile shows, as expected, a distinct E-layer maximum at about 100 km, before falling off to a minimum value around 200 km. The density then increases again to form the F-layer peak. The temperature, on the other hand, increases more or less monotonically with altitude. Primary particle measurements from the NEED-l rocket show that there was a more or less continuous precipitation of aurora1 particles during the whole flight. The particle spectra exhibited the “usual” auroral signature with an intensity peak at some keV, moving to higher energies (more than 25 keV) at apogee before decreasing again in the latter half of the flight. The general intensity increased as well during that period of time. It should also be noted that the precipitating particles were clearly field-aligned, especially at energies around a few keV, for significant parts of the flight. Since the rocket at the time of apogee passage was travelling through the same volume of space as that monitored by EISCAT, the measurements obtained around this time should, in principle, be directly comparable to each other. In Fig. 4 the electron temperature obtained from the rocket measurements (dots) are compared to the electron temperature obtained from the EISCAT measurements (crosses) at different altitudes. As is seen, there is a better agreement between the two curves at lower altitudes, where the measurements were obtained from different regions, than around the apogee. There, the in-situ measurements indicated an electron temperature of more than 3000 K at that time. The data obtained from the ground, on the other hand, yielded a corresponding value of around 2000 K. Even though these measurements were obtained by very different
Electron heating in the ionospheric F-Layer
L
RANGE
(km)
Te f@3 2600 2450 2300 2150 2000 1850 ??OO t5sO WOO 1250 1100 950 800
Ne fm”)
1. OE+t2
i
FIG, 2. ELECTRON TEMPERATURE (K),w PART,PLOTIEDASPUNCTIONOFDISTANCE
Tm
6. 8E+11 4. 6E+ll 3. 2E+lf 2, 2E+ll 1. 5E+lf f. OE+lf 6. 8E+ f0 4.6E+lO 3. 2E+lO 2. ZE+lO 1. 5E+lO 1. OE+lO
UPPER PARC, AND ELECTRON DENXTY (m-‘), IN 17% LOWER (km) FROMT~AAWTENNADURINGTHITEVENT 7 NOVEMBER, 1988. Intensity scales are given to the right of the respective plots. The elevation angle of the antenna was 60.3”.
907
Electron heating in the ionospheric F-layer I
I
I
I
NEED-II flight as well. During that flight the ionospheric conditions were rather quiet with only a small number of precipitating particles present. In this case the measurements carried out from the rocket agreed very well with the EISCAT data. Thus, it seems likely that the discrepancies between the rocket and ground observations during the NEED-I flight were related to the plasma conditions inside the particle precipitation region rather than any instrumental effects. Consequently, on the basis of the previous argument, it seems reasonable to direct the investigation of these measurements towards the mechanism which can most readily be related to the actual plasma environment during crossings of precipitation regions. Hence, in the following we will turn our attention to the question of the influence of suprathermal electron populations on the electron temperature measurements.
4. INTERPRETATION
OF THE AMBIENT
ELECTRON
MEASUREMENTS
i---r-x
i
CLC
Log
Ne.
Te. eV
cm-3
FIG. 3. ELECTRON DENSITY (cm') IN THE LEFT PART AND ELECTRON TEMPERATURE (ev) IN THE RIGHT PART, PLOlTED AS FUNCTIONS
OF ALTITUDE.
Data were obtained by the Langmuir probe onboard the NEED-I rocket.
techniques, a discrepancy of a factor of two warrants some attention. Several comparisons of rocket and incoherent scatter measurements of the electron temperature have been performed previously, see e.g. Oyama and Schlegel (1988) and references therein. These have resulted in diverging conclusions with the in-situ measurements sometimes agreeing with the remote measurements, but just as often turning out to be significantly higher than the ground measurements. Various explanations have been put forward to explain these differences, including electron temperature anisotropies, probe surface phenomena and the influence of non-Maxwellian velocity distributions. In this connection, it is of interest to note that another comparison of electron temperature measurements from rockets and from EISCAT has already been presented by Schlegel and Oyama (1987). These authors concluded that the measurements were in good agreement outside an aurora1 arc encountered by the rocket late in the flight. This is in agreement with our experience from the
Since it was expected that the electron population encountered during this flight might be highly nonthermal, the rocket was equipped with a suprathermal electron spectrometer (SES) mounted on a 16 cm long boom. This instrument was operative at altitudes above about 90 km both on the upleg and the downleg. However, for a short period on the upleg, an electronic malfunction rendered the measurements from the height interval between about 150 and 220 km useless. The same event also disabled part of the dynamic range of the sensors, but still left the instrument capable of obtaining measurements with a high enough quality to give a fairly good representation of the ambient electron population. The instrument consists of two directional electron probes with time-varying retarding potential analyzers. These were swept up and down in voltage between - 14.5 and + 1.5 V, relative to the SES structure, resulting in a time resolution of 0.2 s for these measurements. This corresponded to a spatial resolution of about 71 m at the time of the apogee passage. The whole SES structure was biased to + 1.8 V relative to the payload in order to reduce the effects of any negative vehicle charging. A schematic diagram of the sensor is shown in Fig. 5. In order to appreciate the significance of this design, it is important to understand the consequences of some of the details of the particular analysis technique used in this experiment. The two electron probes were both built into a metallic sensor box with a circular aperture defining the opening angle of each probe. The electrons would then enter the analyzer through
908
K. R. SVENES et al.
200-
0
Rc.4.
1000
X -
Te, EISCAT
.
Te, ROCKET
2000
-
3000
ELECTRON TEMPERATUREMEASUREMENTSOBTAINED
4000
DURING THE NEED-I
K
CAMPAIGN.
The curve marked by crosses is based on incoherent scatter radar measurements (EISCAT), the curve marked by dots is based on Langmuir probe measurements from the rocket. Notice the large differences in the temperature measurements (ca 1000 K) at the apogee point of the trajectory. This is especially noteworthy since these measurements, in contrast to lower altitudes, were obtained from the same volume of space.
a combination of their own momentum and the influence of the electric field induced by the bias potential of the sensor box. Once inside the box the electrons are immediately subjected to an analysis field in order to differentiate between their various energies. At this point, however, these electrons are no longer in contact with the ambient plasma. Hence, their energy spectra will always reflect both the distribution
SIMPLIFIED
DIAGRAM
inherent to the plasma itself and the result of its interaction with the sensor box. The above argument applies regardless of whether the sensor box is positive or negative relative to the plasma potential. However, the interpretation of the resulting energy spectra is crucially dependent on this relationship. If the box is at a positive potential relative to the plasma, the resulting energy spectrum will
OF SES - SENSOR ROCKET STRUCTURE
RETARDING
\ -14.5
I II
POTENTIAL
+ 1.5
/
INDIVIDUALLY (SWEEPING) BIASED
BAFFLES
FIG. 5.A SCHEMATIC DIAGRAM OFTHESES-SENSOR BOX. The electrons enter the sensor from the left, and are then submitted to an analysis field inside the box. Since the box in effect acts as a Faraday cage, the electrons have no contact with the surrounding plasma once they are inside the sensor. The limited size of the opening in the box gives the sensor an effective opening angle of about 60”.
Electron heating in the ionospheric F-iayer
exhibit the characteristics of the internal distribution of the electron population augmented by the energy imparted to it by the ~s~~vely biased box. On the 5th hand, if&e box is negative relative to the @asma onfy those efeetrons with enough energy to penetrate the sheath surrounding the box will then be able to enter it and subsequently become analyzed there. Hence, the resultmg spectrum is then in reality only a part of the total electron d~s~bu~~on of the ambient piasma. Usually9 a precise knowledge of fhe bias applied to the sensor box (relative to the payload) and the charging of the main payload (relative to the plasma) will enable us to determine the effective potential of the SES relative to the plasma potential. Unfore
F-REGION
nA
&mate@, &ring the ftight of NEED-I Sxse criteria were not fulfilled. The languor probe did not seem
to indicate any major charging of the payload structure during the flight. However, certain problems with the interpretation of these instruments at present seem to indicate that no unique rocket ground potential conid be established. In addition, the same electronic ma~fun&t~on which hampered the SES-meas~~m~nts during part of the flight probably also left the sensor box in a state where its bias relative to the payload was indeterminate. However, there exists circumstantial evidence which may still guide us to fairly reasonable canciusions aborts the So-m~su~ments~ This is in part, based on the certain properties of integral current measurements and partly on supporting measurements by the Langmuir probe. It is well known that, while the sweep potential is in the repulsive regime of the plasma electrons and the electrons are assumed io be Ma~we~~~an~~t~bnt~, the resulting cnrrentvoltage curves represent the farnihar Boftzmann relation for electrons. ‘This interpretation was first. described by Langmuir and Mott-Smith Jr (1924) in detail. By comparing the actual measured curves with estimated distributions, based on the above assumptions and using plasma parameters obtained by the Langmuir prtllx, it should be possiMeP in principle, to draw some canclusions about the electron spectra obtained by the SES as well. The most striking feature of the SES measurements is the fundamental difference between data obtained in the F-layer and the E-layer. This is illustrated in Fig. 6. fn this figure the actual data sets ~~~~~~js~~~ are plotted together with estimated distributions (triangles) in order to relate the measurements indirectly to the ambient plasma parameters. The lower part shows a spectrum obtained at an altitude of 120 km. This is representative of the measurements inside the E-layer both on the upieg and the downleg. The ear-
E-REGION nA 4.0
3.5 1 :
*=2.
I 3.0
*@tm-’
T=llWK
1
t 1
ev
0.D 0.0
0.2
0.4
0.6
0.8
1.5
1.2
1.4
FIG. 6. TWO GLECTRON ENERGY SPECTRA OBTAINED I>UBfNG FLIGHT OF NEED-1 COMPARED WITH CORRESmNDING
THE
SiMUu~D
SPECTRA.
marked by after&k& wtiie triangfcs. 1~ the upgxx oa~t ofthe figure is dis&aved data From the Flayer obtained it an altitudeof 250 km, and in the lower part data from the E-layer obtained at an altitude of 120 km. The important point to note here is that the spectrum obtained in the F* layer indicates a much wider electron energy distribution than the one from the E-layer.
The
act&
m~swd
the sinmlated
spxti
spectraare
zs%
givenby
responding simulated spectrum is based on the assumption that the electron population was Maxwellian distributed with a temperature of 1100 K and a density of 2 x 10” m- 3 (as measured by the Langmuir probe].
PLO
K. R. Scenes et at.
Comparing this with the spectrum displayed in the upper part of Fig. 6 immediately focuses on the qualitative difference between the two cases. This spectrum was obtained at an altitude of 250 km and is typical for the m~su~men~ made during the whoie passage through the F-layer. The corresponding simulated spectrum is this time based on the assumption that the electron population was bi-Maxwellian distributed with a thermal population having a temperature of 2500 K and a density of I x 10” me3 (again as measured by the Langmuir probe). In order to describe the high-energy part of this spectrum, it was necessary to introduce a suprathermal tail as well, having a temperature of about 6500 K and a density of about 5% of the thermal population. This was done on the basis of a best tit to the data points. The fact that only very tow values (currents) were actually observed in the low-energy portion of the spectrum is best understood by assuming that the sensor box was at an effective negative potential relative to the plasma during this part of the flight. This is based on the following argument. First suppose that the spectrum in the upper part of Fig. 6 realty represented a thermal electron distribution. Fitting this spectrum to a Maxwellian distribution would then yield a plasma density some two orders of magnitude lower than that which was observed both by the Langmuir probe and by EISCAT. This is in itself a highly unlikely interpretation. As pointed out above, there is also the matter of the considerable energy broadening of the spectra when comparing sweeps obtained in the F-layer with those obtained in the E-layer. The fact that the spectrum in the upper part of Fig. 6 contains electrons with up to 0.5 eY more energy than the spectrum in the lower part argues that it is the high-energy portion of the electron distribution which now dominates the measurements (remember that these are integral measurements). It is also noteworthy that the Langmuir probe and the SES yielded similar results in the E-layer on both upleg and downleg, thereby indicating that the SES low-energy measurements functioned properly also during the passage through the Flayer. Hence, we feel justified in assuming that the spectra obtained here represent the suprathermal part of the ambient electron distribution. Adopting this view, it is then possible to monitor the temporal development of these measurements for characteristic trends in the suprathermal electron population. In Fig. 7 we have plotted the angle between the upward pointing SES sensor and the local geomagnetic field (upper part) and the total current measured during each sweep by that sensor (iower part) for a period of time during the downleg.
The altitude was somewhat more than 300 km at this point. It is evident that the current was modulated by the angle between the sensor and the geomagnetic geld. The maximum current was always obtained when the sensor was most field-aligned. This was a regular feature during that part of the tlight in which the rocket was outside the E-layer. Furthermore, the same trend was seen in the data collected by the downward-looking sensor. However, these currents were always smaller than those collected by the upward-looking sensor. In Fact, the ratio of the current in the upwardlooking sensor to that of the downward-looking sensor amounted to more than 2 at times. Thus, if we assume as argued above, that the measurements are indicative of a suprathermal eketron population, we are left with the following interpretation. There seems to have been created a quite significant field-aligned suprathermal electron population in a source region somewhere above the rocket apogee (which was at an altitude of 322 km). Based on the current intensities measured by the SES, it seems likely that this population, at times, must have reached about 5% of the density of the regular thermal electron population in the P-region. 5. DISCUSSION
AND CONCLUSION
Based on the assumption that the SES was in fact able to measure a part of the suprathermal electron population inside the F-layer, we are now in a position to reconcile several of the other observations made during the NEED-I flight. These data may be explicable by any of several models when viewed separately, but we feel that the existence of a suprathermal electron component in the ambient plasma constitutes the only mechanism able to inliuence all of these measurements in a self-consistent manner. First of all we have the huge differences in the currents collected by the SES sensors in the F-layer and the E-layer (illustrated in Fig. 6). This was probably due to sensors being charged negatively retative to the plasma potential when the rocket was outside the relatively high density plasma in the E-layer, and consequently only the high energy part of the electron spectrum was then measured. The alternative explanation would have been that the SES measured a significantly enhanced electron temperature and a plasma density almost two orders of magnitude lower than that which was observed by both EXSCAT and the ion probe on the same rocket. This explanation seems highly unlikely since the SES and the ion probe made very similar measurements in the E-layer both on the upleg and the dowmeg.
911
Electron heating in the ionospheric F-layer
350.000
351.000
352.000
354.000
353.000
..__......... 2.505-10 ~ .._.__._____._.....................................................................~.......................................... ~
i.SOE-10 350.000
I 351.OOo
FIG. 7. IN THE UPPER PART IS SHOWN
”
352.000
I
”
”
353.000
THE ANGLE BETWEEN THE UPWARD LOCALGEOMAGNETICFIELDLINE.
POINTING
i
35r1.000 SES SENSOR
AND THE
In the lower part is shown the total current measured during each sweep of that sensor. Both of these parameters are plotted as functions of time of flight, and the corresponding data were obtained in the Flayer at an altitude of about 300 km.
Secondly, there is the discrepancy between the EISCAT and the in-situ electron temperature measurements around the time of the rocket apogee passage (shown in Fig. 4). At this time the observations from both the ground and the rocket were in the same volume of plasma, but the electron temperature measured from the rocket was still well in excess of that observed by the EISCAT radar. As mentioned previously, this kind of discrepancy has been observed several times before and tentatively attributed to various mechanisms. However, Hoegy (197 1) treated this problem theoretically and showed that large temperature differences would occur in the presence of significant suprathermal electron populations. In such a situation the temperature deduced from probes would be the higher, as was indeed the case during the flight of the NEED-I rocket. Finally, there is also the question of the enhanced heating rates during this event, noted by Svenes et al. (1992). Since the observed ionosphere electron temperature increase during the flight was significantly higher than that which could be accounted for by collisional heating from the precipitating particles, the extra energy dissipation in the F-region was here tentatively ascribed to some sort of wave-particle
interaction. Such interactions would of course leave traces in the form of an enhanced suprathermal electron population. Furthermore, wave modes likely to be involved in such interactions, like Langmuir waves or the field-aligned mode of the lower hybrid waves, have repeatedly been observed in the aurora1 region, see e.g. McFadden et al. (1986), Bering et al. (1987) and references therein. In such cases the amount of suprathermal electrons would have been greatest parallel to the geomagnetic field lines. This too, is in accordance with the NEED-I observations. In conclusion, we feel that there is enough evidence to establish that a significant suprathermal electron population was present in the F-region during the flight of the NEED-I rocket. Unfortunately, there were no wave measurements to confirm any correlation with the particle measurements. However, as shown above, there is circumstantial evidence for assuming that the suprathermal electron populations may have been the result of wave-particle interactions. In any case, this population could very well be a key to the understanding of the mechanisms responsible for the energy dissipation obviously occurring inside the F-layer during aurora1 particle precipitation. Thus, the study of this question would
K. R. Svar*IES et
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probably help to explain several interesting energy transfer mechanisms active in the ionospheric plasma and should, as such, warrant further work both theoretically and experimentally. Acknowledgements-The authors appreciate the excellent work of the payload integration crew headed by 0. Kristiansen. This project was financially supported by the Royal Norwegian Council for Scientific Research.
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